832 Prof. Carl Barns on the 



Table I. 

 Quartz plate normal to axis. 



5996 cm. i = r = 0°. 



Sunlight. 





Remarks. 



AN. 



Short collimator. Ellipses centred 



•3-1025 

 •34000 

 •34020 

 •34040 

 •34015 

 •34050 

 •34040 

 •34060 



No collimator. Sunlight and slit. Ellipses not centred ... 

 Do. Ellipses centred, but distorted ... 

 Do. Ellipses centred 



Weak objective as collimator. Ellipses less distorted 



Do. with large telescope 



Short collimator. Sharp ellipses 



Concave lens added to collimator 





The beam, in case o£ sunlight, is sheet-like, with its long 

 dimension vertical. Much of it is lost over the edge of 

 mirrors, on successive reflexion. The arc lamp and a colli- 

 mator objective which places the focus (relatively to the 

 vertical plane) on the opaque mirrors M and N furnish a 

 rapier-like beam. This may easily be made to penetrate 

 tubes and columns as a whole. It is thus far preferable and 

 the results better. The sodium line is invariably present, as 

 a line of reference for measurement. 



3. Dispersion Constants. — In the equation in the normal 

 form (4) for two wave-lengths X and V, b may be eliminated 

 or determined; for if ?/= sini = (normal incidence, detected 

 with certainty by the inversion in the motion of the rings as 

 described above), a* = AN/«, and /jL = A + bj\ 2 is assumed for a 

 sufficiently narrow range of the spectrum near the D line, 



A=^ a (l + ^X 2 -^V)/(^-V), ... (6) 



36 = M 2 (a'-^)/(1-(XAi) 2 ) (7) 



If the value of oe is restored, since for the two wave-lengths 



AN = N'-N and AN^Ni'-l^, 



where W refers to glass and N to air, 



e (^ 1 _^)=N 1 '-N l -(N'-N)=N 1 / -N / -(N 1 -N), 



or briefly e(xi— #) = SN'— SN", 



whence 



b = 



pX SN'-SN 

 3e l-(VXiP 



(8) 



Here BW and SN are the differences of micrometer readings 

 for the two spectrum lines X and X', for glass in place and 

 for glass removed, respectively. AN X and AN need not 



